Engine systems may be configured with boosting devices, such as turbochargers, for providing a boosted aircharge and improving peak power outputs, fuel economy and emissions. Power from exhaust gas may be used to rotate the impeller blades of the turbine which in turn drives an intake compressor. By adjusting exhaust manifold flow through the turbine, higher boost pressures can be achieved, enabling more fresh air to be introduced into the engine cylinders. As a result, soot emissions are reduced and transient engine output power is improved.
A portion of the exhaust gas from upstream and/or downstream of the turbine may also be recirculated to an engine intake system, in a process referred to as exhaust gas recirculation (EGR), in order to effectively cool the combustion chamber, thereby reducing NOx formation and improving fuel efficiency. High pressure EGR (HP EGR) includes exhaust drawn from upstream of the turbine, and therefore is driven by the same exhaust manifold flow as an exhaust turbine. Since exhaust turbine and EGR systems share the same exhaust manifold, there may be interaction issues between the EGR passage and turbine passage which may result in an unsatisfactory transient response for both EGR flow and boost pressure.
Accordingly, various approaches have been developed to coordinate the operation of an exhaust turbine and an EGR system. In one example, as shown in U.S. Pat. No. 9,133,795, Riley et al. teaches a split exhaust system including each of a high pressure exhaust valve in communication with an exhaust turbine via a high pressure exhaust manifold and a low pressure exhaust valve in communication with an intake manifold turbine via a low pressure exhaust manifold. On an exhaust stroke, an initial pulse of high pressure exhaust may be delivered to the turbine via the high pressure exhaust valve and the high pressure exhaust manifold and then the remaining exhaust may be delivered on a subsequent pulse to the intake manifold as EGR via the low pressure exhaust valve and the low pressure exhaust manifold.
However, the inventors herein have recognized potential issues with such systems. As one example, during lower engine load conditions, when boost demand is lower while high pressure EGR demand is higher, it may not be possible to deliver the desired high pressure EGR via the low pressure exhaust valve and the low pressure exhaust manifold. As another example, during transient increases in engine load, such as during a tip-in, the exhaust flow via the turbine may not be sufficient to provide the desired boost pressure, thereby adversely affecting the engine's transient response. As still another example, during conditions when boost pressure and EGR is desired, an increase in the intake pressure relative to the exhaust pressure due to turbine operation can cause high pressure EGR flow from the exhaust manifold to the intake manifold to be significantly reduced. The issue can be exacerbated in turbochargers with electric assist where an electric motor is coupled to the turbocharger drive shaft to transiently increase the compressor output during a tip-in.
In one example, the issues described above may be addressed by a method for an engine comprising: operating an intake compressor with each of motor torque from an electric motor and mechanical torque from an exhaust turbine, flowing a first portion of exhaust from a cylinder to upstream of the compressor via a first exhaust valve while flowing a second, remaining portion of the cylinder exhaust to the turbine, and operating in a first mode with an opening and closing of the first exhaust valve advanced relative to an opening and closing of the second exhaust valve. In this way, by using a turbocharger with an electric assist device and by flowing exhaust to the turbine and to the intake manifold (as EGR) via distinct exhaust passages controlled by separate exhaust valves, desired boost pressure may be maintained while delivering the requested amount of EGR.
In one example, the exhaust manifold may be divided into two independent exhaust manifolds, each coupled to a distinct exhaust valve. A first exhaust manifold, coupled to a first exhaust valve, may deliver a first portion of exhaust to an exhaust turbine of a turbocharger, such as a variable geometry turbine (VGT), while a second exhaust manifold, coupled to a second exhaust valve, may deliver a second portion of exhaust to upstream of an intake compressor of the turbocharger as EGR. The ratio of exhaust flow via the first exhaust manifold (to turbine) relative to the second exhaust manifold (as EGR) may be adjusted by adjusting exhaust valve profiles (valve timing and valve lift) for the first exhaust valve and the second exhaust valve. In one example configuration, the turbocharger may be an electric turbocharger having an electric motor coupled to the turbocharger shaft between the exhaust turbine and the intake compressor. In an alternate configuration, the turbocharger may be included in a compound boosted system having an electric supercharger coupled to a bypass passage of the intake manifold, upstream of the turbocharger compressor. During conditions of increased boost demand, in addition to the boost provided by the exhaust turbine, the electric motor may be operated to provide electric boost assistance wherein positive motor torque is delivered to the intake compressor in order to meet the requested boost demand and EGR demand. During conditions of lower boost demand, the electric motor may be operated as a generator storing excess exhaust energy wherein negative motor torque is provided to decelerate the compressor and charge a system battery. Also, vanes of the VGT or the opening of a waste-gate passage coupled across the exhaust turbine may be adjusted to provide the desired boost pressure and EGR flow. During conditions when a boost error (that is, difference between target boost pressure and actual boost pressure) is higher relative to an EGR error (that is, difference between target EGR flow and actual EGR flow), the exhaust valve profiles may be adjusted to route a higher amount of exhaust via the turbine while reducing the amount of exhaust available as EGR. During conditions when the EGR error is higher relative to the boost error, the exhaust valve profiles may be adjusted to recirculate a higher amount of exhaust to upstream of the compressor while reducing the amount of exhaust routed to the exhaust turbine. Further, an opening of an EGR valve coupled to the EGR passage delivering exhaust from the second exhaust manifold to the intake manifold may be adjusted based on EGR demand. The number of cylinders supplying exhaust to the exhaust turbine and for EGR may also be adjusted based on boost error and EGR error.
In this way, by using a split exhaust system having distinct exhaust manifolds for supplying exhaust to upstream of the turbine and the intake compressor, interaction between the two exhaust flows may be reduced, thereby improving boost pressure and EGR delivery during transient engine operations. By using different exhaust valve profiles for controlling exhaust flow via the distinct exhaust manifolds, the cylinder pressure may be effectively used to drive exhaust flow to each of the exhaust turbine and the intake manifold. The technical effect of using an electric motor coupled to a shaft of the turbocharger is that in addition to reducing boost error, the electric motor may create a pressure difference across the EGR valve and operate as an EGR pump to improve EGR flow during transient operations. By opportunistically adjusting each of the duration of exhaust valve lift, the timing of exhaust valve lift, and the number of cylinders supplying exhaust to the turbine and the intake manifold, exhaust energy may be optimally used for providing boost while maintaining EGR supply during transient engine operations. Overall, by improving boost and EGR supply, fuel efficiency and emissions quality may be improved.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.